Method for the purification of an alpha-hydroxy acid on an industrial scale

ABSTRACT

The invention relates to a method for the purification of a fermentatively prepared α-hydroxy acid on an industrial scale, in which an α-hydroxy acid with a color (fresh) of not more than 10,000 APHA units is subjected to at least two crystallization steps, with the crystallization steps being carried out in cooling crystallization devices, melting crystallization devices, evaporative crystallization devices and/or adiabatic crystallization devices.

The present invention relates to a method for the purification ofα-hydroxy acids, in particular lactic acid or glycolic acid, on anindustrial scale, as well as to products of the utmost chiral puritywhich can be obtained by this method, and to applications thereof.

Lactic acid is usually marketed as a dilute or concentrated solution,because lactic acid has a strong tendency to form intermolecular esters(dimeric and polymeric lactic acid). In addition, lactic acid (even verypure lactic acid) is strongly hygroscopic. The purification of lacticacid (the racemic mixture and in particular the enantiomers of lacticacid) on an industrial scale is a complicated and difficult processaccording to the prior art.

It is known how to produce lactic acid, or 2-hydroxypropionic acid, in afermentative manner. In general the fermentative production of lacticacid includes first of all a fermentation step in which acarbohydrate-containing substrate such as glucose or sucrose isconverted to lactic acid by a suitable microorganism. Knownmicroorganisms producing S-lactic acid are various bacteria of the genusLactobacillus, such as Lactobacillus casei for example. In additionmicroorganisms are also known which produce (R)-lactic acid selectively.The aqueous fermentation product is then processed in order to obtainlactic acid. The usual industrial processing path generally consists ofseparation of the biomass followed by acidification, purification andconcentration.

In the case of (S)-lactic acid the lactic acid so obtained issufficiently pure to be processed in foods for human consumption. (S)-or (R)-lactic acid which is ultimately obtained by this usual method canbe 98% enantiomerically pure or even higher (i.e. 98% or more of thelactic acid present consists of the (S)- or (R)-enantiomer). The productstill contains residual sugars, however. The product is also yellow incolour and on heating this becomes brown to black through decompositionof impurities. Moreover, in the case of (S)-lactic acid, theorganoleptic properties often leave something to be desired. The lacticacid enantiomer is thus moderately suitable for application in foods,but on the whole not suitable for pharmaceutical applications and forsynthesis of compounds.

The purity of the product can be increased by esterification followed byhydrolysis, so that it is suitable for pharmaceutical applications. As aresult of this esterification/hydrolysis, however, the enantionmericpurity decreases and the lactic acid still contains a small amount ofthe alcohol which has been used in the esterification. Examples of othermethods for the purification of lactic acid include subjecting aqueoussolutions of lactic acid to one or more extraction, (steam) distillationand/or evaporation steps, electodialysis steps and crystallizaions (seefor example Ullmans Encyklopädie der Technischen Chemie, Verlag ChemieGmbH, Weinheim, fourth edition, Part 17, pages 1-7 (1979); H. Benninga,“History of Lactic Acid Making”, Kluwer Academic Publishers,Dordrecht-Boston-London (1990); C. H. Holten, “Lactic Acid; Propertiesand Chemistry of Lactic Acid and Derivatives”, Verlag Chemie GmbH,Weinheim (1971); The Merck Index, Merck & Co., Inc., eleventh edition,page 842 (1989); Römmp Chemie Lexicon, G. Thieme Verlag, Stuttgart andN.Y., ninth edition, Part 4, pages 2792-2893 (1991) and the Netherlandspatent applications 1013265 and 1013682.

In German Patent 593,657 (granted on 15 Feb. 1934) a laboratoryexperiment is described in which an aqueous solution of lactic acid,which contained an excess of the S component and practially no lacticacid anhydride, was concentrated by means of a thin-film evaporationtechnique, if necessary at reduced pressure. The concentrated lacticacid solution was then rapidly cooled, with formation of crystals. Afterthat the crystals were separated from the mother liquor, washed withether and repeatedly recrystallized from ethyl acetate or chloroform ora comparable solvent until the crystals showed a sharp melting point of53° C. The chiral purity of the enantiomeric excess and the colour arenot reported.

In H. Borsook, H. M. Huffman, Y-P. Liu, J. Biol. Chem. 102, 449-460(1933) a laboratory experiment is described in which an aqueous mixture,which contained 50 percent lactic acid with an excess of (S)-lacticacid, 30 percent lactic acid anhydride and lactic acid dimer and 15percent water, was subjected to fractional distillation at approximately0.13 mbar and 105° C. The middle fraction was the distilled again andafter that cooled in an ice/salt bath with formation of a solid crystalmass. It is reported that the distilliation has to be preformed withsmall quantities, because with larger quantities there is a big loss ofproduct as a result of the long heating time. The solid crystal mass wasthen recrystallized three times from an equal volume of equal quantitiesof diethyl ether and diisopropyl ether, and the crystals were isolatedand dried at room temperature in a vaccum drier. In this way it waspossible to obtain (S)-lactic acid with a melting point of 52.7-52.8° C.which contained less than 0.1 percent impurities such as water, lacticacid anhydride or lactic acid dimer. The chiral purity or theenantomeric excess and the colour of (S)-lactic acid are not reported.

In L. B. Lockwood, D. E. Yoder, M. Zienty, Ann N.Y. Acad. Sci. 119, 854(1965) the distillation and crystallization of lactic acid on alaboratory scale is also descibed, the melting point of the opticallypure lactic acid obtained being 54° C. The colour is not reported.

In 1934 the crystallization of lactic acid was investigated byBoehringer Ingelheim, but this method was not found to give goodresults, owing to problems with the purification and further treatment.After the Second World War, however, it turned out that BoehringerIngelheim was able to produce lactic acid for pharmaceuticalapplications on a scale of about 12 to 15 tons per month, with a yieldof about 77 to 86 percent. In this process an aqueous solution of lacticacid was purified by means of steam distillation at reduced pressure(abount 13 mbar), followed by crystallization at −25° C., after whichthe crystals were dissolved in water and the solution was treated withpotassium ferrocyanide (to remove heavy metals) and activated charcoal.The chiral purity or the enantiomeric excess or other properties such ascolour and odour of the (S)-lactic acid so produced are not known (seeH. Benninga, “History of Lactic Acid Making”, Kluwer AcademicPublishers, Dordrecht-Boston-London, pages 347-350 (1990)).

Crystalline (S)-lactic acid has been marketed by, for example, Fluka andSigma with purities of more than 99% (see for example M. L. Buszko, E.R. Andrew, Mol. Phys. 76, 83-87 (1992) and T. S. Ing, A. W. Yu, V.Nagaraja, N. A. Amin, S. Ayache, V. C. Gandhi, J. T. Daugirdas, Int. J.Artif. Organs 17, 70-73 (1994)). Crystallic S-lactic acid with a watercontent of less than 1 percent by weight is known from EP A 563,455 (seeExample 1). The crystal structure of lactic acid is described in A.Schouten, J. A. Kanters, J. van Krieksn, J. Mol. Stuct. 323, 165-168(1994). Lactic acid can also be obtained in a synthetic manner. This isknown. The product of the synthetic production method, however, is aracemic mixture which thus contains (S)-lactic acid and (R)-lactic acidin equal quantities. It is true that the separate enantiomers can beseparated by means of known techniques, such as diastereoisomerseparation techniques, where one of the enantiomers crystallizes out asa salt and this salt is then converted back to the enantiomeric lacidacid, but the enantiomeric product finally obtained will inevitablystill contain significant quantities of the other enantiomer.

In European Patent Application 552,255 it is reported that glycolic acidof industrial quality can be crystallized by putting a solution thereofin a freezer, giving rise to crystals which are filtered off. It will beclear that such a method is unsuitable for being carried out on anindutrial scale. Such a method is also applied in DE A 2,810,975.

In WO 00/56693 a method is described for the purification of lactic acidon an industrial scale, the method involving: (a) the distillation underreduced pressure of a concentrated lactic acid solution with a totalacid content of at least 95% by weight and a enantiomeric lactic acidcontent of at least 80% by weight, calculated in terms of theconcentrated lactic acid solution, and with a ratio of the lactic acidenantiomers not equal to 1, and (b) subjecting the distilled lactic acidsollution to a crystallization, with formation of pure lactic acid,where the pure lactic acid has a total acid content of at least 99% byweight, a monomeric lactic acid content of at least 98% by weight, achiral purity of 99% or more, calculated in terms of the total quantityof pure lactic acid, a colour of not more than 10 APHA units and anacceptable odour.

In EP A 733,616 a method for the production of glycolic acid crystals isdescribed in which an aqueous solution of glycolic acid is concentrated,seed crystals are added to the concentrated solution and the solution iscooled.

Disadvantages of the method according to WO 00/56693 are that inparticular step (a) of this method requires a great deal of energy andthat complicated distillation equipment is required.

The present invention aims to solve this problem and therefore relatesto a method for the purification of an α-hydroxy acid on an industrialscale (i.e. a scale of at least 10000 tons per annum), in which anα-hydroxy acid with a colour (fresh) of not more than 10,000 APHA unitsis subjected to at least two crystallization steps.

Advantages of the present invention are that the method requires littleenergy and relatively simple equipment can be used.

An α-hydroxy acid means a carbonic acid which is substituted with ahydroxy group on the α carbon atom. The general formula of an α-hydroxyacid is therefore:

where R is a hydrogen atom, a C₁-C₅ alkyl group (preferably a methylgroup), a C₆-C₁₂ aryl group or a heterocyclic cycloalky or -aryl group.The α-hydroxy acid according to the invention is preferably lactic acid(R is methyl) or glycolic acid (R is hydrogen) and is in particularlactic acid.

The feed for the method is preferably characterized by a colour (fresh)of not more than 7500 APHA and in particular of not more than 5000 APHA,a total acid content of at least 70% by weight, relative to the wholefeed, and a free acid content of at least 60% by weight, relative to thewhole feed. If the α-hydroxy acid is lactic acid, the feed preferablyhas a total acid content of at least 80% by weight and a free acidcontent of not more that 10,000 ppm, preferably not more than 5,000 ppm,and a total quantity of residual sugars (predominantly polysaccharides)of not more than 20,000 ppm, preferably not more than 10,000 ppm, whereall the contents here indicated are relative to the whole feed. Thechiral purity of the feed, if applicable, is at least 90% and preferablyat least 95%.

Total acid content (TA) is the acid content after saponification ofintermolecular ester bonds with an excess base and is determined by backtitration with acid. The total acid content thus gives the quantity ofmonomeric, dimeric and polymeric lactic acid. The free acid content (FA)is determined by direct titration with base, i.e. before saponificationof the intermolecular ester groups. The content of monomeric lactic acid(MM) is here as defined as:MM=TA−2×(TA−FA)provided that TA−FA <10%. This means that not very much dimeric orpolymeric lactic acid can b present. It is also assumed that thenon-monomeric lactic acid is present in the form of lactoyl lactic acid(dimer).

Chiral purity (for an excess S-isomer) is here defined as:Chiral purity=100% ×{(S-isomer)/(R-isomer+S-isomer)}

According to the invention two crystallization steps are preferablycarried out, with both crystallization steps preferably being carriedout in one device.

The known crystallization techniques can in principle be applied in themethod according to the present invention. An example of such atechnique is melting crystallization (or cooling crystallization), wherethe condensed, liquid concentrate or distillate, which for examplecontains (S)- or (R)-lactic acid in a molten state, is directly cooled,so that the (S)- or (R)-lactic acid crystallizes out. It is preferableto keep the temperature at which crystallization occurs (thecrystallation temperature) as low as possible, so that the formation ofoligomers and polymers of the α-hydroxy acid is limited as much aspossible. According to the invention a concentrate is preferably used,since the preparation of a distillate is unfavourable in terms ofprocess energy.

Melting crystallization is a process in which a crystalline materialobtained from a melt of the material to be crystallized. This techniqueis for example described in detail in Kirk-Othmer, Encyclopedia ofChemical Technology, fourth edition, Part 7, pages 723-727 (1993), in J.W. Mullin, “Crystallization”, third revised edition.Butterworth-Heinemann Ltd., pages 309-323 (1993) and in J. Ullrich andB. Kallies, Current Topics in Crystal Growth Research, 1 (1994), whichhave been recorded here for reference. The main advantages of meltingcrystallization relative to distillation is that much less energy isneeded, because the enthalpy of melting of organic compounds isgenerally lower than the enthalpy of evaporation. This advantage alsooccurs with other crystallization techniques, because the enthalpy ofcrystallization relative to distillation is furthermore that the processcan generally be carried out at a much lower temperature—which isadvantageous when the organic compound is thermally unstable.

The melting crystallization can be carried out with the aid of asuspension crystallization or a layer crystallization, if necessary incombination with a washing column or a centrifuge, or anotherpurification technique. Examples of suitable equipment and process aredescribed in Kirk-Othmer, Encyclopedia of Chemical Technology, fourthedition, Part 7, pages 723-727 (1993), in J. W. Mullin,“Crystallization”, third revised edition, Butterworth-Heinemann Ltd.,pages 309-323 (1993) and J. Ullrich and B. Kallies, Current Topics inCrystal Growth Research, 1 (1994), the content of which has beenrecorded here for reference.

It has also been found that crystallization of an aqueous solution givesvery good results. In this crystallization treatment a concentratedlactic acid solution is for example diluted with water and this is thensubjected to one or more cooling and/or evaporative crystallizationsteps. In these techniques the concentrate or distillate is directlycooled (cooling crystallization) or concentrated by evaporation of water(evaporated crystallization). The driving force for the crystallizationin the cooling crystallization techniques is the bringing about ofsupersaturation in the concentrated lactic acid solution by reducing thetemperature of the concentrated lactic acid solution. As a resolution ofthe lower temperature of the solution the solubility decreases andsupersaturation occurs.

The driving force for the crystallization in the evaporativecrystallization technique is the bringing about of supersaturation in,for example, a concentrated latic acid solution by evaporation of water,so that the concentration of the solution increases while thetemperature remains constant. Crystallization of the lactic acid thenoccurs during the evaporation of water.

Another highly suitable crystallization technique is adiabaticcrystallization, where the driving force for the crystallization is thebringing about of supersaturation in, for example, a concentrated lacticacid solution by evaporation of water without supplying heat. Theevaporation of water has two effets: (a) the temperature of theconcentrated lactic acid solution becomes lower and (b) theconcentration of the acid increases. Both effects lead to a decrease inthe solubility and an increase in the supersaturation.

The crystallization steps are preferably carried out according to theinvention by means of adiabatic crystallization or coolingcrystallization, in particular by means of adiabatic crystallization.Seed crystals are preferably added to the feed in the crystallizations.If a solvent is used in the crystallization, this is preferably water.

The α-hydroxy acid which is crystallized out can then be separated bythe known methods for solid-liquid separation from the remaining liquid,or mother liquour.

Examples of suitable separation techniques for separating the α-hydroxyacid crystals from the mother liquor are centifugation, decanting,filtration, separation by means of one or more washing columns, or acombination of two or more of these techniques. In the context of theinvention it has been found that centrifugation and separation with oneor more washing columns is particularly appropriate.

The mother liquors which are obtained still contained considerablequantities of α-hydroxy acid. For optimal process management it istherefore preferable to feed these mother liquors back into the process.

After isolation, the α-hydroxy acid crystals which are obtained aredirectly dissolved in suitable solvent, usually water, in order toprevent coagulation of the hygroscopic α-hydroxy acid crystalsoccurring. The concentration of the α-hydroxy acid solution so obtainedcan in principle have any desired concentration. In practice this willusually vary form 30 to 95%. Concentrations commonly occurring on themarket are 80-90%.

The invention also relates to an α-hydroxy acid or an α-hydroxy acidsolution with a chiral purity of at least 99% and a colour of not morethan 20 APHA units, preferably not more than 10 APHA units, with theα-hydroxy acid or the α-hydroxy acid solution having an acceptableodour, in particular for pharmaceutical applications. In the case of anα-hydroxy acid solution the solvent is preferably water. The chiralpurity is preferably at least 99%, in particular at least 99.5%, whichcorresponds to 99% enantiomeric excess (ee) or higher. Most preferableis chiral α-hydroxy acid, or the solution thereof, whose chiral purityis at least 99.8% (i.e. at least 99.6% ee).

The α-hydroxy acid or the α-hydroxy acid solution also meets thefollowing requirements:

-   -   alcohol content: not more than 250 ppm (alcohol is methanol,        ethanol or other alcohol, as alcohol as such or in the form of a        lactate).    -   total nitrogen: not more than 15 ppm.    -   total sugar: not more than 100 ppm.    -   total polysaccharides: not more than 100 ppm.    -   organic acids (other than lactic acid): not more than 250 ppm.

With regard to odour the α-hydroxy acid or the α-hydroxy acid solutionpossesses a considerable improvement for application in foods and ahigher chemical purity than the products according to the prior art.

When it is chiral, the α-hydroxy acid according to the invention can beboth an S-α-hydroxy acid and a (R)-α-hydroxy acid, depending on themicroorganism which is used in the fermentation.

Because of their high chiral purity both the (S)-α-hydroxy acid and the(R)-α-hydroxy acid or the solutions thereof can very suitably be appliedfor chiral syntheses. The chirally pure (S)-α-hydroxy acid or solutionsthereof are also very suitable for being applied in pharmaceuticalpreparations.

The invention therefore also relates to a pharmaceutical preparationwhich contains the (S)-α-hydroxy acid or the (S)-α-hydroxy acid solutiondescribed above. The invention is now illustrated by means of thefollowing example.

EXAMPLE

(S)-lactic acid with the following properties is used as the startingmaterial:

Total acid content  95.4% Free acid content  91.1% Colour (fresh) 4850APHA Total nitrogen 1080 ppm Total residual sugars 6490 ppm Chiralpurity 99.61%

In the first crystallization step a double-walled 2.7 litre vessel wasconnected with a thermoblast bath and 2045 g of the starting materialdescribed above was put into the vessel. The acid was cooled to 40° C.while stirring and inoculated with 0.4 g of a suspension which containedseed crystals. The acid was then cooled from 40° to 30° C. in 5 hours inaccordance with a linear colling programme. The crystals formed wererod-shaped and many small particles were formed. After 5 hours thetemperature of the thermostat bath was 30° C. and that of the crystalsuspension of the acid was 31.9° C. The suspension was centrifuged(Sieva laboratory centrifuge, Hermle). 831 g of crystals and 1061 g ofmother liquor was obtained (yield of 46%, calculated in terms of lacticacid).

In a second crystallization step a three-necked round-bottomed flask of0.5 litre was placed in a thermostat bath and 349 g of the crystalsobtained above and 22.3 g of water were put into the flask, so that asuspension was obtained which corresponded to a lactic acidconcentration of about 94%. The suspension was heated while stirring inorder to dissolve all the crystals and the solution was then cooled to36° C. while stirring. After that about 0.27 g of a suspension whichcontained seed crystals was added and the seed crystals were left togrow for 10 minutes at 36° to 24° C. The mixture was then cooled inaccordance with a linear programme (from 36° C. to 24° C. in 6 hours).The crystals formed had a cuboid shape. After cooling to 24° C. thesuspension was centrifuged: 165 g of crystals were obtained from 356 gof suspension (yield of 49%, calculated in terms of lactic acid). Thetotal yield from the two crystallization steps was 22%, calculated interms of lactic acid. The crystals from the first and secondcrystallization were dissolved in water (90% solution) and the solutionswere analysed. The results are shown in the table below.

First crystallization Second crystallization Colour (fresh) 349 APHA 14APHA Colour (after heating) 713 APHA 18 APHA Total nitrogen  55 ppm <10ppm Total residual sugars 430 ppm   40 ppm^(a) Chiral purity99.97% >99.99% ^(a)This value is close to the detection limit.

1. Method for the purification of a fermentatively prepared α-hydroxyacid on an industrial scale, in which an α-hydroxy acid with a colour(fresh) of not more than 10,000 APHA units is subjected to at least twocrystallization steps, with the crystallization steps being carried outwith water as a solvent or without any solvent in coolingcrystallization devices, melting crystallization devices, evaporativecrystallization devices and/or adiabatic crystallization devices. 2.Method according to claim 1, in which the α-hydroxy acid is lactic acidor glycolic acid.
 3. Method according to claim 2, in which the α-hydroxyacid is lactic acid.
 4. Method according to claim 1, in which twocrystallization steps are applied.
 5. Method according to claim 4, inwhich the crystallization steps are carried out in one device.
 6. Methodaccording to claim 1, in which the product stream from thecrystallization steps is separated into a mother liquor and α-hydroxyacid crystals by means of solid-liquid separation.
 7. Method accordingto claim 1, in which, if applicable, the chiral purity of the α-hydroxyacid is at least 90% with a colour (fresh) of not more than 10,000 APHAunits.
 8. Method according to claim 1, in which the α-hydroxy acid isprepared fermentatively, with a colour (fresh) of not more than 3000APHA units.
 9. Method according to claim 1, in which the coolingcrystallization is carried out by direct cooling of a condensed, liquidconcentrate or distillate of the α-hydroxy acid.
 10. Method according toclaim 1, in which the melting crystallization is carried out by directcooling of a condensed, liquid concentrate or distillate which containsthe α-hydroxy acid in a molten state.
 11. Method according to claim 1,in which the evaporative crystallization is carried out by (1) dilutinga condensed, liquid concentrate or distillate of the α-hydroxy acid withwater and (2) evaporation of water.
 12. Method according to claim 1, inwhich the adiabatic crystallization is carried out by evaporation ofwater without supplying heat.